1. Field of the Invention
The present invention relates to printing, and more particularly, to a method for manufacturing a printing plate. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a method for manufacturing a printing plate that can form a fine pattern.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices, which have image quality equivalent to that of a cathode ray tube, are used in a wide variety of applications because of their advantage of light-weight, thin profile, and compact size. In general, a liquid crystal display device includes an array substrate, a color filter substrate and liquid crystal molecules between the substrates such that pixels in a matrix are respectively controlled to display images. The array substrate has a plurality of gate lines and data lines crossing each other to define pixel areas, pixel electrodes made of a transparent metal respectively formed in the pixel areas and TFTs serving as switching units for the pixel electrodes, and a color filter substrate having a transparent insulative substrate, a black matrix layer, and RGB color filter layers formed on the transparent insulative substrate opposite to pixel electrodes of the array substrate. The array substrate and the color filter substrate are bonded to each with liquid crystal molecules interposed therebetween.
The array substrate and the color filter substrate are independently manufactured. Before the array substrate and the color filter substrate are bonded to each other, an orientation film depositing step, a rubbing step, a spacer distributing step, and a seal printing step are performed. When these steps are finished, the array substrate and the color filter substrate are positioned opposite to each other, and then bonded to each other by applying heat and/or irradiating ultraviolet rays.
The seal printing step is performed on the array substrate to hermetically seal a space between the two substrates to prevent the liquid crystal molecules from flowing out of the space when the liquid crystal molecules are injected into the space. Further, the seal printing step bonds the two substrates to each other. The seal printing step can be performed by using one of four different methods.
The first method is to form a seal pattern by screen printing, which uses simple production equipment and efficiently utilizes the sealing material. Screen printing uses a mask having a patterned screen, which is spaced from the upper surface of a substrate by a designated interval, and then a paste required to form a seal pattern is compressed and transcribed onto the substrate through the patterned screen so that a desired seal pattern is formed on the substrate. Screen printing is being used in the manufacture of LCDs and plasma display panels (PDPs).
Generally, a seal pattern having a height of approximately 20 μm is formed by a screen printing step prior to a baking step to dry the seal pattern. To form a seal pattern having a height of 50˜100 μm, the screen printing steps are repeated five times to ten times with baking steps in between to dry a newly printed seal pattern. The repeated printing and baking steps to form a thick seal pattern decrease the productivity of the liquid crystal display. Due to alignment variances over the course of the repeated printing and baking steps, a seal pattern with a thin profile is difficult to obtain. Further, reproducibility in terms of achieving a desired height with a desired number of repeated printing and baking steps is not consistent.
The second method is to selectively sand blast sealing material that has been spread on the substrate to form the desired seal pattern. The sand blast method is used to form a fine seal pattern in the manufacturing of a large-sized panel. For example, sealing material is printed over the whole surface of a substrate having electrodes formed thereon using a screen printing method, a photosensitive film is applied to the sealing material, and only portions of the photosensitive film for protecting the sealing material are left on the sealing material through an exposure and development process. Then, an abrading agent is sprayed at the sealing material on the substrate to remove portions of the sealing material, which are not protected by the photosensitive film. Al2O3, SiC, or ultrafine particles of glass can be used as the abrading agent, and the abrading agent can be sprayed by using compressed air or nitrogen gas.
The sand blast method is used to form a sealing pattern having a height of less than 70 μm on a large-sized glass substrate. The sand blast method mechanical impacts the substrate with the abrading material such that microscopic damage can occur in the substrate that later develop into cracks in the substrate during baking. Further, the sand blast method raises production costs due to consumption of many materials uses costly equipment. In addition, the sand blast method is complicated and causes dust pollution.
The third method is to spray the seal pattern directly onto a substrate by dispensing the sealing material with pressurized air pressure through a template. The dispenser method eliminates the costs of using a photoresist mask and a seal pattern can be deposited as a thick film because the sealant material starts drying while airborne. Further, the dispenser method is a simple procedure and can be used for applying a seal pattern in large-sized LCDs and PDPs.
The fourth method is to plate print the seal pattern. FIGS. 1A to 1C are cross-sectional views illustrating a printing process for forming a set of patterns on a substrate according to the related art. As shown in FIG. 1A, a pattern material 20 is applied to a printing roll 10 using a printing nozzle 30.
As shown in FIG. 1B, the printing roll 10, to which the pattern material 20 is applied, is applied to a printing plate 40, in which a designated figure is engraved. Then, a part 20b of the pattern material 20 is transcribed on protrusions of the printing plate 40, and the other part 20a of the pattern material 20 remains on the printing roll 10.
As shown in FIG. 1C, the printing roll 10 having the remaining pattern material 20a then is rotated on a substrate 50, thereby transcribing the remaining pattern material 20a on the substrate 50.
A plate printing apparatus can be used to form letters and designs on a wrapping paper. However, the plate printing apparatus may be used for other purposes, such as formation of a thin film. For example, the plate printing apparatus can be used to form an orientation film of a liquid crystal display device by printing a polyimide thin film on a glass plate, or to form a seal pattern for a liquid crystal panel. Hereinafter, with reference to the accompanying drawings, a related art method for manufacturing a printing plate for a plate printing apparatus will be described.
FIGS. 2A to 2E are cross-sectional views illustrating a method for manufacturing a printing plate according to the related art. As shown in FIG. 2A, a metal film 52 for a hard mask is deposited on an insulative substrate 51, and a photoresist 53 then is applied to the metal film 52. The metal film 52 is made of a metal, such as Cr or Mo. Subsequently, the photoresist 53 is selectively patterned through photolithography process, including exposure, thereby defining pattern regions.
As shown in FIG. 2B, the metal film 52 is selectively removed using the patterned photoresist 53 as a mask, thereby forming a metal film pattern 52a (or hardmask).
As shown in FIG. 2C, the photoresist 53 is removed from the insulative substrate 51. The removal of the photoresist 53, which is used as a mask for forming the metal film pattern 52a, is performed by a method using oxygen gas plasma or a method using an oxidizer. In the oxygen gas plasma method, oxygen gas is injected onto a substrate under a vacuum and a high-voltage bias over the substrate generates an oxygen gas plasma that reacts with the photoresist to remove the photoresist by decomposition.
As shown in FIG. 2D, the insulative substrate 51 is selectively etched using the metal film pattern 52a as a mask, thus forming trenches 54 having a depth of approximately 20 μm into the surface of the insulative substrate 51. Isotropic etching using a HF-group etchant can be performed on the insulative substrate 51.
As shown in FIG. 2E, the metal film pattern 52a is removed from the insulative substrate 51.
The printing plate, which is manufactured by the above method, is used in the printing apparatus of FIG. 1B. Then, a desired printing material is coated on the printing roll, the printing material on the printing roll is selectively printed on the printing plate, and the printing material on the printing plate is transcribed onto the object to be printed, thus producing the desired pattern.
The above related art method for manufacturing the printing plate has disadvantages. For example, since the trenches having a desired depth are simultaneously formed in the insulative substrate by etching the insulative substrate using the metal film pattern as a mask, the etching critical dimension (CD) increases due to the characteristics of isotropic etching, thus causing a difficulty in manufacturing a fine printing plate. In other words, the width of an etch increases faster than the depth of an etch during etching. In general, the width of an etched trench is at least twice as much as the depth of an etched trench. Thus, when the etch depth into the insulative substrate is 5 μm, it is impossible to form a line width (A of FIG. 2D) of less than 10 μm.